1. Technical Field
The present invention relates to silicon-germanium (SiGe) epitaxial (EPI) growth, and more particularly, to yield improvement in silicon-germanium epitaxial growth.
2. Related Art
A typical fabrication process to form an NPN device starts out with a single-crystal silicon layer on a wafer. Then, first and second shallow trench isolation (STI) regions are formed in the single-crystal silicon layer. The collector of the NPN device is to reside in a first single-crystal silicon region sandwiched between the first and second STI regions. Next, silicon (Si) and germanium (Ge) are deposited on top surfaces of the single-crystal silicon region and the first and second STI regions. As a result of the SiGe deposition, a second single-crystal silicon region grows up from the top surface of the first single-crystal silicon region. Also as a result of the SiGe deposition, first and second polysilicon regions grow up from the top surfaces of the first and second STI regions, respectively. The emitter and base of the NPN device are to reside in the second single-crystal silicon region.
The first single-crystal silicon region and the first STI region have a first common interface surface. The first single-crystal silicon region and the second STI region have a second common interface surface. The top edges of the first and second common interface surfaces are where high material stress is present. This high material stress may result in cracks that propagate into the first and second single-crystal silicon regions causing electrical shorts between the collector and the emitter of the NPN device. This in turn decreases the yield for the NPN device.
Therefore, there is a need for a method for fabricating NPN devices having a relatively higher yield than is disclosed in the prior art.